The purpose of this proposed study is to understand how mothers manage breastfeeding infants with the disorder of phenylketonuria (PKU). PKU is an inborn error of metabolism disorder. Approximately 160-400 infants per year are born with PKU in the United States. The incorporation of breastfeeding into the management of PKU has not been well studied. This results in a lack of specific breastfeeding guidelines for mothers in this population. Specific aims are: 1) describe how mothers manage breastfeeding in the context of PKU, and 2) identify patterns of management of breastfeeding based upon the Family Management Style Framework (FMSF). The FMSF has been used to describe family management of a number of chronic conditions as well as lactation management in families of premature infants. The first aim will identify the day-to-day management experiences of mothers'of infants with PKU who are breastfeeding and are required to supplement with phenylalanine-free formula. The second aim addresses how mothers simultaneously manage breastfeeding and the responsibilities and activities of PKU therapy. This aim will identify patterns of how mothers in the PKU population learn to manage breastfeeding and the responsibilities and activities of PKU therapy based upon the FMSF. The proposed study uses a predominantly qualitative approach to provide an understanding of mothers'experiences of breastfeeding infants with PKU. In addition, to more fully comprehend the subject matter and to provide a more comprehensive view of the family, two measures will be used to collect data on parenting stress and family functioning. Qualitative interviews will be the primary data collection technique. Data from the standardized measures on parental stress and family functioning will be used to situate breastfeeding management within the broader context of overall family life. Data will be collected from a sample of 25-40 mothers of breastfeeding infants with PKU. Mothers will be recruited from across the United States. This proposed study will contribute to establishing a scientific basis for the care of breastfeeding infants with PKU. LAY LANGUAGE: The purpose of this study is to understand how mothers manage breastfeeding an infant with the disorder of phenylketonuria (PKU). PKU is an inborn error of metabolism disorder. The incorporation of breastfeeding into the management of PKU has not been well studied. This study will identify and describe the day-to-day management experiences of mothers who are breastfeeding infants with PKU and identify the factors that contribute to the experiences of mothers'success in breastfeeding their infants with PKU while adhering to the PKU management plan.